In conventional engine manufacture, the size of the cylinder block is normally dictated by the capacity of the cylinder bores. In particular, the surface area of the top deck of the block is affected by the diameter of each of the cylinder bores. As a result, increasing the capacity of a cylinder block by increasing the diameter of the cylinder bores requires a larger and heavier cylinder block to accommodate the larger bores. This increase in the size and weight of the block will negate to a certain extent the improvement in performance provided by the increased engine capacity created by the larger diameter bores.
As a result of this disadvantage, engine manufacturers have attempted to obtain greater cylinder bore dimensions, and hence engine cubic capacity, within an engine block without substantially adding to the size and weight of the block itself. The disadvantage of such arrangements is that increasing the bore diameters without lengthening the block means that the space between the end walls of the block and the walls of the outermost cylinder bores becomes limited. As a water jacket must be located between the cylinder bores and the end walls, the transverse portions of the water jacket between the end walls and outermost bores must be thinner than usual because of the reduction in space.
As will be understood by those skilled in the art, the conventional way in which to define a water jacket during cylinder block casting is to use moulded sand cores in the block mould. However, if the transverse portions of the water jacket between the end walls and outermost bores are too thin, the thinner sand cores needed to define the thinner transverse portions of the water jacket may not be strong enough during casting. If the cores are too thin they may tend to crack or deform. Thus, efficient block casting of compact but increased capacity blocks remains difficult.
A cast cylinder block is provided with a variety of internal volumes, apertures and recesses that define various elements within the block itself. In conventional engine block casting, the shape or profile of such internal features is dictated by the shape of sand cores which are pre-moulded and placed within a cylinder block mould prior to the metal being cast into the mould. These cores themselves are shaped in core boxes, which are conventionally split into two parts, with the split between the two parts at either the top or bottom of the box in order that the formed cores may be removed. However, the shape that the cores can be formed in—and hence the shape of the internal features in the cylinder block—is limited, as the cores must be easily removed from the core box prior to insertion into the cylinder block mould. With the split in the core box at either the top or bottom of the box, the cores must only taper longitudinally in one direction if they are still to be easily removed from the core box.
This problem of core shape is especially significant when considering the profile of a water jacket for a cylinder block, where the water jacket is positioned between the side wall of the block and the cylinder bores. As the cores can only taper in one direction, the water jacket created by the core also only tapers in one direction, narrowing when viewed in transverse section from the top deck of the block downwards. This presents problems in that the water jacket cannot be particularly deep given the single taper, and the cylinder bores must also be relatively far apart so that there is room on the deck of the block for machining additional features. Furthermore, with a water jacket which is wider at the top of the block the wall thickness between the bore and jacket will be relatively thin, which is not desired when the combustion—and hence greatest heat transfer—occurs at the top of the cylinder bore.
Conventional cylinder blocks are also cast such that the water jackets are closed at the top thereof. This is disadvantageous in the manufacturing process as it prevents easy cleaning and inspection of the block after both casting and machining.
It is an aim of the present invention to obviate or mitigate one or more of the aforementioned problems.